Sn or Sn alloy coatings such as Sn, Sn—Cu, Sn—Ag, Sn—Bi and the like have been widely used in electronic parts and the like because of their excellent solder wettability. It is known that if these plated coatings have an internal stress therein, a hair-shaped crystal called whisker is liable to occur, with the attendant problem that circuits are short-circuited by the action of the whiskers. A Sn alloy-plated coating has a better effect of inhibiting whisker generation than a Sn-plated coating, but not satisfactory.
In order to inhibit the whisker generation, the following methods (see Technical Reports of Mitsubishi Electric Corporation, 1979, vol. 53, No. 11 (Non-Patent Document 1)) have been hitherto used, but with individual problems.
(1) To carry out Ni plating on a Sn and Sn alloy-plated undercoat: serving as a barrier for the formation of an intermetallic compound between a Cu material and plated Sn thereby inhibiting whisker generation. In this regard, however, there are a number of parts that are unable to be plated with Ni because of the characteristics required therefor and such Ni plating is not effective against contact pressure-induced whiskers generated by the action of an external stress.(2) To make a thick plated coating of Sn or an Sn alloy plating (10 to 20 μm or over): if the coating is made thick, an influence of an internal stress caused by the formation of an intermetallic compound does not extend over a plated surface layer, thereby inhibiting whisker generation. In this connection, however, there are some types of electronic parts which cannot be applied thereon with a thick coating.(3) To carry out a thermal treatment after Sn and Sn alloy plating: when the thermal treatment is carried out, an internal stress caused by the formation of an intermetallic compound is mitigated, thereby inhibiting whisker generation. Usually, treated under about 150° C.-one hour. Because of the thermal treatment after plating, this is not effective against contact pressure-induced whiskers applied with an external stress after the thermal treatment.(4) To carry out reflowing after Sn and Sn alloy plating: an Sn coating is completely melted, for which reflowing is more effective than thermal treatment. However, if a plated coating is thick, a uniform coating cannot be obtained after reflowing. An oxide film is formed on the Sn coating by reflowing, thereby degrading solder wettability. Reflowing is not effective against contact pressure-induced whiskers applied with an external stress after reflowing.
Even with a member that is reduced in internal stress and is unlikely to form whiskers on its own, if a stress is exerted thereon from outside, contact pressure-induced whiskers generate. The contact pressure used herein means an external load exerted on a Sn or Sn alloy coating in case where a flexible substrate formed with the Sn or Sn alloy coating is fitted into a an insertion portion of a contact of electronic parts such as, for example, a connector and the like and also an external load added to a Sn or Sn alloy coating when other member is fixed to the Sn or Sn alloy coating by pressure welding. Contact pressure-induced whiskers are generated on a plated coating of Sn or Sn alloys such as Sn, Sn—Cu, Sn—Ag, Sn—Bi and the like, which is formed as a surface treatment at the fitted portion or pressure-welded portion of electronic parts.
As a method for preventing contact pressure-induced whiskers from occurring, mention is made of a method of inhibiting contact pressure-induced whiskers from being generated by carrying out a thermal treatment under conditions of an external pressure being imposed thereon such as after fitting, thereby mitigating a stress caused by the external pressure (Japanese Patent Laid-Open No. 2005-154835 (Patent Document 1)). Among actual electronic parts, some parts involve a difficulty in carrying out a thermal treatment after fitting and thus, its application may not be possible depending on the type of electronic part. No effective method for such electronic parts has been found yet.
Patent Document 1:    Japanese Patent Laid-Open No. 2005-154835
Patent Document 2:    Japanese Patent Laid-Open No. 2002-317295
Patent Document 3:    Japanese Patent Laid-Open No. Hei 10-102266
Patent Document 4:    Japanese Patent No. 2942476
Patent Document 5:    Japanese Patent Publication No. Sho 56-47955
Patent Document 6:    Japanese Patent Publication No. Sho 56-47956
Patent Document 7:    Japanese Patent Laid-Open No. 2002-53981
Patent Document 8:    Japanese Patent Laid-Open No. Sho 63-285943
Non-Patent-Document 1:    Technical Reports of Mitsubishi Electric Corporation, 1979, vol. 53, No. 11